The importance of clean semiconductor workpiece surfaces in the fabrication of semiconductor microelectronic devices has been recognized for a considerable period of time. Over time, as VLSI and ULSI silicon circuit technology has developed, the cleaning processes have gradually become a particularly critical step in the fabrication process. It has been estimated that over 50% of the yield losses sustained in the fabrication process are a direct result of workpiece contaminants. Trace impurities, such as sodium ions, metals, and particles, are especially detrimental if present on semiconductor surfaces during high-temperature processing because they may spread and diffuse into the semiconductor workpiece and thereby alter the electrical characteristics of the devices formed in the workpiece. Similar requirements are placed on other such items in the electronics industry, such as in the manufacture of flat panel displays, hard disk media, CD glass, and other such workpieces.
Cleaning of a semiconductor workpiece, and other electronic workpieces, occurs at many intermediate stages of the fabrication process. Cleaning of the workpiece is often critical after, for example, photoresist stripping and/or ashing. This is particularly true where the stripping and/or ashing process immediately proceeds a thermal process. Complete removal of the ashed photoresist or the photoresist/stripper is necessary to insure the integrity of subsequent processes.
The actual stripping of photoresist from the workpiece is yet another fabrication process that is important to integrated circuit yield, and the yield of other workpiece types. It is during the stripping process that a substantial majority of the photoresist is removed or otherwise disengaged from the surface of the semiconductor workpiece. If the stripping agent is not completely effective, photoresist may remain bonded to the surface. Such bonded photoresist may be extremely difficult to remove during a subsequent cleaning operation and thereby impact the ability to further process the workpiece.
Various techniques are used for stripping photoresist from the semiconductor workpiece. Mixtures of sulfuric acid and hydrogen peroxide at elevated temperatures are commonly used. However; such mixtures are unsuitable for stripping photoresist from wafers on which metals, such as aluminum or copper, have been deposited This is due to the fact that such solutions will attack the metals as well as the photoresist. Solvent chemistries are often used after metal layers have been deposited. In either case, limited bath life, expensive chemistries, and high waste disposal costs have made alternative strip chemistries attractive.
Plasma stripping systems provide such an alternative and have been used for stripping both pre- and post-metal photoresist layers. This stripping technique, however, does not provide an ideal solution due to the high molecular temperatures generated at the semiconductor workpiece surface. Additionally, since photoresist is not purely a hydrocarbon (i.e., it generally contains elements other than hydrogen and carbon), residual compounds may be left behind after the plasma strip. Such residual compounds must then the removed in a subsequent wet clean.
Ozone has been used in various applications in the semiconductor industry for a number of years. Often, the ozone is combined with deionized water to form an effective treatment solution The attractive features of such a solution include low-cost, repeatable processing, minimal attack on underlying device layers, and the elimination of waste streams that must be treated before disposal. The main drawback with using such solutions has been the slow reaction rates that translate into long process times and flow throughput.
Photoresist strip using ozone dissolved in water has been somewhat more successful in achieving viable process rate at acceptable process temperatures. However, ozone, like all gases, has a limited solubility in aqueous solutions. At temperatures near ambient, ozone saturation occurs at around 20 ppm. Ozone solubility in water increases dramatically with decreasing temperature, to a maximum of a little over 100 ppm at temperatures approaching 0 degrees Celsius and drops to almost zero at temperatures approaching 60 degrees Celsius. While increasing ozone concentration increases the kinetic reaction rate, a decrease in temperature simultaneously suppresses that rate.
A technique for stripping photoresist and/or cleaning a semiconductor workpiece using ozone and deionized water is set forth in U.S. Pat. No. 5,464,480, titled xe2x80x9cProcess and Apparatus for the Treatment of Semiconductor Wafers in a Fluidxe2x80x9d, issued Nov. 7, 1995. The ""480 patent purports to set forth a method and apparatus in which low-temperature deionized water is ozonated by bubbling ozone through the low-temperature water. The low-temperature, ozonated, deionized water is in the form of a bath. Semiconductor wafers are batch processed by immersing the wafers in the bath, for example, to clean the wafers, strip photoresist, etc.
The present inventors have found that the foregoing system purportedly described in the ""480 patent may not be optimal for use in many circumstances. Static boundary regions between the bath and the surface of the semiconductor workpiece may result in sub-optimal cleaning and/or stripping. Finally, ozone concentration in the deionized water bath may be difficult to maintain in view of the fact that the apparatus of the ""480 patent is an open atmospheric system.
An apparatus for supplying a mixture of a treatment liquid and ozone for treatment of a surface of a workpiece, such as a semiconductor workpiece, and a corresponding method are set forth. The preferred embodiment of the apparatus comprises a liquid supply line that is used to provide fluid communication between a reservoir containing the treatment liquid and a treatment chamber housing the semiconductor workpiece. A heater is disposed to heat the workpiece, either directly or indirectly. Preferably, the workpiece is heated by heating the treatment liquid that is supplied to the workpiece. One or more nozzles accept the treatment liquid from the liquid supply line and spray it onto the surface of the workpiece while an ozone generator provides ozone into an environment containing the workpiece.
Generally, a heated treatment liquid is ill suited for dissolving ozone therein. As such, a thick boundary layer of treatment fluid disposed on the surface of the workpiece may act to inhibit the ability of the ozone to reach and react with the components that are, for example, to be removed from the surface of the workpiece. The apparatus therefore preferably includes one or more processing components that are used to control the thickness of the boundary layer of the heated treatment liquid on the surface of the workpiece. Reducing the thickness of the boundary layer facilitates diffusion of the ozone through the boundary layer to the surface of the workpiece. Significantly increased cleaning and stripping rates have been observed in such an apparatus, particularly when the treatment liquid is a water-containing liquid such as deionized water.
In accordance with a preferred method for treating a workpiece, the workpiece is first heated. A treatment liquid is provided to the surface of the workpiece that is to be treated and an amount of ozone is introduced into an environment containing the workpiece. Even more preferably, the thickness of a liquid boundary layer on the surface of the semiconductor workpiece is controlled to allow diffusion of the ozone therethrough so that the ozone may react at the surface of the workpiece.
In accordance with yet another embodiment of the apparatus, the apparatus comprises a liquid reservoir having a liquid chamber, a pump having an input in fluid communication with the liquid chamber and an output in fluid communication with one or more nozzles disposed to spray fluid therefrom onto the surface of the workpiece. A fluid path extends between the output of the pump and the nozzle and carries the pressurized liquid that is provided at the output of the pump. An ozone supply system injects ozone into the fluid path. As such, a pressurize mixture of treatment liquid and ozone is sprayed onto the surface of the semiconductor workpiece to thereby eliminate many of the problems associated with prior systems. A method for treating a workpiece in the manner exercised by the foregoing apparatus system is also disclosed.